/Engine/Source/Developer/MaterialUtilities/Public/MaterialUtilities.h
TODO replace this with rendering property enum when extending the system
enum EFlattenMaterialProperties { Diffuse, Metallic, Specular, Roughness, Anisotropy, Normal, Tangent, Opacity, Emissive, SubSurface, OpacityMask, AmbientOcclusion = 16, NumFlattenMaterialProperties, }